1. Field of the Invention
The invention relates to the field of ellipsometry and, more particularly, to a novel ellipsometer which determines uniquely and unambiguously the phase difference .DELTA. between the parallel (R.sub.p) and perpendicular (R.sub.s) components of a beam which has been elliptically polarized by reflection from a sample whose properties are to be measured, while at the same time uniquely determining the ellipsometric parameter .psi. of the elliptically polarized beam.
2. Description of the Prior Art
Ellipsometers per se are well known in the prior art. In classical ellipsometry, the beam is passed through two manually adjustable polarizing elements. The polarizing elements are adjusted to produce a null output at a detector upon which the beam impinges. By measuring the relative angular positions of the polarizer elements at which a null is produced, the two ellipsometric parameters .DELTA. and .psi. can be determined, where .DELTA. is the phase difference between the parallel and perpendicular components R.sub.p and R.sub.s, respectively, of the electric vector of the reflected beam, and tan .psi. = (R.sub.p /R.sub.s). From these parameters two unknown properties of the optical system under measurement can be determined, i.e., if the elliptically polarized beam is produced by reflection of a linearly polarized beam from the surface of a film, then the film thickness and refractive index can be determined. If the beam is reflected from a bulk sample, then the complex refractive index of the bulk sample can be determined. In other words, in general, the polarization transfer function of the optical system can be determined.
The outstanding feature of ellipsometry has proven to be the ability to measure the thickness of arbitrarily thin transparent films from which a plane polarized beam is reflected to produce the elliptically polarized beam. However, a disadvantage of the classical manually-adjustable system is that the manual search for the null is a slow process typically requiring a measurement time of 10 minutes; therefore, several methods of automating the measuring procedure have been developed.
One of these automatic ellipsometric systems is disclosed in the above co-pending Application Ser. No. 373,540, filed by us for an Automatic Ellipsometer, on June 25, 1973, now U.S. Pat. No. 3,880,524, and assigned to the assignee of the present application. In this co-pending Application, the second polarizing element or analyzer is rotated continuously, and the intensity of the transmitted light is monitored as a function of the instantaneous rotational angles of the rotating analyzer. From the resulting data, the polarization state of the light can be deduced, and, consequently, the angles .psi. and .DELTA. can be determined by Fourier analysis.
The rotating analyzer technique of said co-pending Application has several desirable features when compared with other approaches to automated ellipsometry. These desirable features include high speed (measurements in 2.0 seconds) and extremely high precision (the polarization azimuth angle .alpha. can be measured with a standard deviation of 0.002.degree.). The precision inherent in the rotating analyzer technique derives partly from the use of a Fourier analysis of the measured light intensity data, the Fourier expansion series containing a constant value plus a sinusoidal component of twice the angular rotation frequency of the analyzer. Thus, random noise in the individual determinations of intensity is effectively averaged out over a full rotation of the analyzer, thereby improving measurement precision.
However, as pointed out above, the rotating analyzer technique has a disadvantage in that it is incapable of distinguishing between complementary polarization states of equal orientation and ellipticity but of opposite handedness, i.e., left- or right-handed polarization states that are otherwise equal. This disadvantage manifests itself in an ambiguity in the deduced value of the phase angle .DELTA., although the angle .psi. is determined unambiguously. More specifically, the angle .psi. varies only between 0.degree. and 90.degree. and therefore is inherently unambiguous, whereas the ambiguity in the phase angle .DELTA. occurs because .DELTA. varies between 0.degree. and 360.degree..
Of course, this ambiguity in .DELTA. may be removed when using the rotating analyzer technique either by performing a second measurement after changing the polarization by a known amount, or by knowing a certain amount of prior knowledge of the specimen being measured. However, the necessity of performing a second measurement detracts from the inherent advantage of the speed of the rotating analyzer technique, and, also, the required amount of prior knowledge may not be available.
Even though there exist some particularly basic alternative polarimetric techniques which do not have this disadvantage, they have not been applied to ellipsometry. For example, Clarke and Grainger have categorized all the basic combinations of a retardation plate and linear analyzer as polarimeters, i.e., not as ellipsometers, and shown that one such combination provides a complete determination of polarizaton state by a single measurement; the combination is a rotating retarder in front of a stationary analyzer. See D. Clarke and J. F. Grainger, Polarized Light and Optical Measurement, (Pergamon, New York 1971).